1. Field of the Invention
The invention relates to a device and a method for assisting a decision whether or not servicing or maintenance of a system whose condition is monitored by means of a condition sensor is to be carried out. Furthermore, the invention relates to a condition sensor which can be used in such a device or in such a method.
2. Background Information
The invention is in the field of condition-based maintenance (CBM). In brief, CBM means that servicing or maintenance is carried out when there is a need for it. The servicing or maintenance is carried out when an indicator or a plurality of indicators show that the monitored equipment or the monitored system will fail or that operating properties, technical properties, functions of the monitored equipment or of the monitored technical system are degrading.
For the purpose of description of the invention and the advantageous refinements thereof, the terms servicing and maintenance will be used synonymously. They form a generic term for any measure which is useful for maintaining or improving the functional readiness of the monitored technical system, for example exchanging components or wearing parts; cleaning contamination, replacing operating fluids, lubrication; replacing or cleaning filters; removing waste products; repairing areas of damage, etc.
Condition-based maintenance was introduced in order to attempt to service or maintain the correct equipment at the correct time. CBM is based on the use of real time data in order to prioritize and optimize maintenance resources. Monitoring the condition of the system is known as “condition monitoring”. A device for monitoring the condition will detect the “health” or the “state of health” of the monitored equipment and act only if maintenance is actually necessary. Developments over the last years have led to technical systems and equipment being provided with extensive measuring technology, and this has given rise, together with improved tools for analyzing condition data, to a situation in which the maintenance personnel are nowadays more able than ever to take the decision as to when is the correct time for carrying out maintenance on pieces of equipment. The condition-based maintenance ideally permits the maintenance personnel to do only what is correct and necessary, which minimizes spare part costs and downtimes of the system during maintenance and the times required for maintenance.
There is some literature about condition-based maintenance which represents the current state of the art in this field and which explains the terms, measures and devices used in this context. This literature includes:                Jarrell D B, Sisk D R & Bond L J: “Prognostics and Condition-based Maintenance (CBM)—a Scientific Crystal Ball”, publication by Pacific Northwest National Laboratory, Richland, Wash., No. PNL-SA-36771 from 2002 with further references;        Publication of the International Atomic Energy Agency IAEA “Implementation Strategies and Tools for Condition-Based Maintenance at Nuclear Power Plants, IAEA-TECDOC-1551, May 2007, with further references;        Memorandum for Secretaries of the Military Departments, by Deputy Under-Secretary of Defence, USA, 25 Nov. 2002: “Policy for Department of Defence condition-based maintenance”.        
From the publications above, distributed, inter alia, via the Internet, it is apparent that different condition sensors can be used for monitoring variables which can indicate the load or wear of a component of a technical system. For example, vibrations, expansion of materials, temperatures, viscosities or other properties of operating fluids, etc. can be monitored.
Examples from the patent literature on condition-based maintenance can be found in DE 103 32 629 A2, DE 101 44 076 A1, EP 08 95 197 B1, DE 31 10 774 A1, DE 10 2005 012 901 B4, DE 102 22 187 A1, DE 101 48 214 C2.
The respectively monitored technical systems can be entirely different; condition-based maintenance has advantages in many technical areas. Examples can be found in the abovementioned references and patent documents. Condition-based maintenance is also of interest, in particular, in the field of aircraft technology since such technology is particularly maintenance-intensive owing to the relevance of the technical systems used to safety.
There is, in particular, a need to use condition-based maintenance on all possible parts of an aircraft which require maintenance.
However, this is a distant objective which cannot yet be implemented owing to the shortcomings of the devices and methods known hitherto for monitoring conditions and performing maintenance.
With aircraft technology there is also the problem of an expected increase in maintenance works since new materials, such as force-flux-compatible composite fiber materials, are being increasingly used in the construction of aircraft in order to save weight. In particular in the case of fiber-reinforced composite materials it is, however, possible for wear phenomena and invisible internal damage to occur in the case of relatively heavy loading, with the result that an exchange of components made of fiber-reinforced composite materials is indicated after specific scopes of operation and degrees of loading before these components fail. It would therefore be desirable to be able to perform condition-based maintenance on as many of such components as possible.
However, the main disadvantage of CBM is that the initial costs of CBM are very high. CBM requires improved measuring technology for the monitored equipment. The costs of adequate measuring technology are usually very high; this also applies specifically to equipment which has already been installed. However, even when pieces of technical equipment and systems with the previously known condition sensors and evaluation technologies are re-developed, high additional costs are incurred, with the result that condition-based maintenance could previously be implemented only on a small number of main components of technical systems.
The invention is based on the object of obtaining assistance in taking a decision with respect to maintenance planning on the basis of signals of sensors which can supply definitive information about a state of health of the monitored system but nevertheless are simpler and more cost effective to manufacture and to integrate into the measuring technology than was previously the case with condition sensors used in CBM.
There is literature on condition monitoring (CBM) with perfect sensors. However, condition monitoring with perfect sensors can be applied to real systems only to a restricted degree. In order to permit condition-based maintenance (CBM) of a system, the system must be equipped with sensors which are capable of determining the state of health thereof as far as possible in real time (see in this respect DIN 44300). In classic CBM implementations, the sensors determine an expected remaining useful life (RUL) and, if appropriate, a reliability factor (confidence level). The calculation of the RUL is very complex depending on the application and increases the implementation costs.
It is known, for example from the abovementioned literature and patent literature, that high to prohibitive costs are incurred by the development of CBM sensors and, above all, the algorithms which calculate the remaining useful life from acquired physical data of the sensors. This is due to the fact that the sensor attempts to determine the most precise remaining useful life possible at any time between the equipment being put into service and its failure. The developer of the sensor does not know how well the calculation of the remaining useful life has to be effective at specific intervals, with the result that he does not have any possibility to optimize. Until now there has been no product or literature relating to the subject of assistance in making a decision with imperfect sensors.